The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to protect the surfaces of such components with an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Coatings that have found wide use for this purpose include diffusion aluminide coatings and overlay coatings such as MCrAlY (where M is iron, nickel and/or cobalt), which may be overcoated with a diffused aluminide coating. During high temperature exposure in air, these coatings form a protective aluminum oxide (alumina) scale that inhibits oxidation of the coating and the underlying substrate. Diffusion aluminide coatings are particularly useful for providing environmental protection to components equipped with internal cooling passages, such as high pressure turbine blades, because aluminides are able to provide environmental protection without significantly reducing the cross-sections of the cooling passages. As known in the art, diffusion aluminide coatings are the result of a reaction with an aluminum-containing composition at the component surface. The reaction forms two distinct zones, an outermost of which is termed an additive layer that contains the environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone containing various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate.
Hot corrosion of gas turbine engine components generally occurs when sulfur and sodium react during combustion to form sodium sulfate (Na.sub.2 SO.sub.4), which condenses on and subsequently attacks the components' surfaces. Sources of sulfur and sodium for hot corrosion reactions include impurities in the fuel being combusted as well as the intake of sodium laden dust and/or ingestion of sea salt. In the latter situation, hot corrosion typically occurs on hot section turbine blades and vanes under conditions where salt deposits on the component surface as a solid or liquid. The salt deposits can break down the protective alumina scale on the aluminide coating, resulting in rapid attack of the coating. Hot corrosion produces a loosely adherent external scale with various internal oxides and sulfides penetrating below the external scale. These products are generally sulfur and sodium compounds with elements present in the alloy and possibly other elements from the environment, such as calcium, magnesium, chlorine, etc. As such, hot corrosion products are distinguishable from oxides that normally form or are deposited on gas turbine engine components as a result of the oxidizing environment to which they are exposed.
Traditionally, aluminide coatings have been completely removed to allow component repair by welding or brazing or to replace damaged coating, after which a new aluminide coating is applied by any suitable aluminizing process. Any hot corrosion products present in the coating are removed with the coating. A disadvantage of completely removing an aluminide coating from a gas turbine engine component is that a portion of the substrate metal is removed with the coating, which significantly shortens the useful life of the component. As a result, new repair technologies have been proposed by which diffusion aluminide coatings are not removed, but instead are rejuvenated to restore the aluminide coating and the environmental protection provided by such coatings. However, coating rejuvenation technologies for turbine blade and vane repair cannot be performed in the presence of hot corrosion products, since any remaining hot corrosion products would result in attack of the rejuvenated coating upon exposure to engine temperatures. Because hot corrosion products have required removal by abrasive grit blasting, rejuvenation technologies have been limited to components that have not been attacked by hot corrosion.
From the above, it can be appreciated that, in order to successfully implement a rejuvenation program for turbine engine components having diffusion aluminide coatings that are exposed to sea salt and other sources of sulfur and sodium, hot corrosion products must be removed without damaging the aluminide coatings. Treatments with caustic solutions in autoclaves have been successfully used to remove oxides of aluminum and nickel from components, but such treatments have not been successful at removing hot corrosion products for the apparent reason that the more complex hot corrosion products are not soluble in caustic solutions. Accordingly, the prior art lacks a process by which hot corrosion products can be completely removed without damaging or removing a diffusion aluminide coating.